Frequency stabilized multivibrator



Sept. 1, 1970 E. A. VITTOZ 3,526,353

FREQUENCY STABILIZED MULTIVIBRATOR Filed July 5, 1968 INVENTDR ERIC )7 vm'oz HTT RNEyS United States Patent US. Cl. 331-111 3 Claims ABSTRACT OF THE DISCLOSURE A transistor circuit provides a compensating voltage to an astable multivibrator in order to stabilize its frequency, at least for variations due to temperature.

The present invention concerns a frequency stabilized multivibrator, comprising an astable circuit provided with at least two transistors interconnected by means of at least one R-C circuit.

It is known that if the feed voltage of multivibrators of this type is low, their frequency depends on the variations of this feed voltage as well as on temperature variations. This is due to the presence of the base-emitter junctions of the transistors in which the voltages at the terminals are not negligible in relation to the feed voltage.

In order to avoid these effects, it has already been proposed to utilize resistances with a suitably chosen temperature coefiicient; this solution however has the disadvantage that it only allows the temperature effect to be compensated and that it necessitates the use of special components.

It has also been proposed to utilize diodes; this solution has the disadvantage that it cannot be used for voltages as low as 1.3 v. and that it does not afford sufficient freedom to allow a perfect compensation to be achieved.

The purpose of the invention is to avoid these disadvantages.

The multivibrator according to the invention is characterized in that it comprises a compensating circuit provided with a transistor and applying a compensating voltage to the astable circuit, in order to stabilize its frequency, at least for variations in temperature.

The drawing illustrates, by way of examples, two embodiments.

FIG. 1 shows the diagram of a first embodiment, relating to a multivibrator having two transistors interconnected by means of two R-C circuits.

FIG. 2 illustrates the diagram of a second embodiment relating to a short pulse multivibrator.

A multivibrator is an essentially non-linear oscillator the operation of which is characterized by relaxation periods (for instance the discharge of a capacity through a resistance) separated by commutation periods. The well known circuit shown in B in FIG. 1 belongs to this category.

It comprises two transistors T T with a common emitter hook-up, and interconnected by means of two R C circuits. The feed voltage U is applied to the collectors, through collector resistances R If the feed voltage U, of this circuit is high, its variations have very little influence on the frequency of the oscillation; the influence of temperature only makes itself felt through the variations of the relaxation time constant R -C.

When the feed voltage is reduced, an ever increasing influence of this voltage on the frequency is observed,

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as well as an increasing supplementary temperature effect.

This is due to the presence of the base-emitter junctions of the transistors T T the terminal voltages of which v are no longer negligible in relation to U,,.

It is known that the repeating frequency may be modified by varying the ratio U,/ U,,. In other words, the frequency f of the mnltivibrator depends on the feed voltage U,,, the temperature T and the regulating voltage U,. This dependence may be characterized by sensibility factors 8,, S and S defined by the equation In the embodiment illustrated in FIG. 1, the astable circuit is combined with a compensating circuit applying a regulating voltage U the variations of which with the temperature and the feed voltage are such that they correct the frequency variations of the astable circuit (by annulling the second member of the above equation).

The compensating circuit illustrated in A in FIG. 1 comprises a control transistor T with a common emitter hook-up having a resistance R in the emitter circuit and a resistance R, in the collector circuit. Its base is fed by a fraction m of the feed voltage which is the same as that of the oscillating circuit. The compensating voltage U is derived at the collector of T and applied to the common point of the reistances R When the temperature increases, the voltage at the terminals of the base-emitter junction of T decreases by ,u volts per degree; the emitter and collector currents of the transistor T increase and U decreases. This reduction allows the compensation of the decrease of the voltage at the terminals of the base-emitter junctions of the transistors T T of the oscillating circuit. The correction may be adjusted by acting on the ratio p=R /R By utilizing the second degree of freedom afforded by the factor m, it is also possible to compensate the elfect of the variations of U,. The two effects will be compensated if and The second embodiment illustrated in FIG. 2 comprises an astable circuit B comprising two transistors T T interconnected by means of three resistances R R and R and a condenser C. This astable circuit, which is destined to generate short pulses, is compensated by a compensating circuit A, of the same type as that of the preceding embodiment, and applying the regulating voltage U to the base of the transistor T by means of a resistance R In certain cases it may be necessary to add one or several diodes in series in the base of the transistor T in order to obtain a higher value of p.; this naturally also causes an increase of the value of the voltage at the terminals of the base-emitter junction.

I claim:

1. A frequency stabilized multivibrator, comprising an astable circuit provided with at least two transistors interconnected by means of at least one R-C circuit; and a compensating circuit comprising four resistances, and a control transistor, two of said resistances being connected in series as voltage dividers between the poles of a feed current source, their common point being connected to the base of said control transistor, a third of said resistances being connected between a collector and one of the poles of said current source, and the fourth resistance being connected between the emitterand the other pole of said current source; variations in temperature producing changes in the operation of said control transistor, thereby causing a compensating voltage derived at the collector of said control transistor to be applied to the base electrode of at least one transistor of said astable circuit, in order to stabilize its frequency.

2. A multivibrator according to claim 1, in which the two transistors of the astable circuit have a common emitter hook-up with the emitters connected to one of the poles of the current source, each collector being connected to the other pole through a resistance and to the base of the other transistor by means of a condenser, each one of the said bases being connected, through a resistance, to the said collector of the transistor of the compensating circuit.

3. A multivibrator according to claim 1, in which the two transistors of the astable circuit are of opposite types, the emitter of a first one of these transistors being connected to one of the poles of the said source and its collector to the base of the second transistor through a resistance, the collector of this second transistor being of the said first transistor of the astable circuit being,

interconnected by means of a resistance.

References Cited UNITED STATES PATENTS 3,239,778 3/1966 Rywak 331-113 3,264,579 8/1966 Marcus 331113 3,377,567 4/1968 Pontius 331-113 3,388,344 6/1968 West 331l13 JOHN KOMINSKI, Primary Examiner U.S. Cl. X.R. 3311l3, 176 

